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From Wes McKinney <wesmck...@gmail.com>
Subject Re: Converting clickhouse column to arrow array
Date Wed, 29 Jan 2020 16:49:19 GMT
On Wed, Jan 29, 2020 at 9:55 AM Calder, Matthew <mcalder@xbktrading.com> wrote:
>
> I managed to get conversion from CH to arrow using a CHToArrowType<> inter-type
traits concept. However, I am still trying to crack the use of:
>
>  arrow::VisitArrayInline

Here's a minimal example of VisitArrayInline

struct ArrayVisitor {
  Status Visit(const Array& arr) {
    return Status::OK();
  }
};

Status VisitArrayInlineExample(const Array& arr) {
  ArrayVisitor visitor;
  return VisitArrayInline(arr, &visitor);
}

You can add different Visit functions to match different specific
Array subclasses or groups of types (e.g. integers, floating point,
etc.). std::enable_if is helpful (and the various helper templates in
arrow/type_traits.h)

>
> and
>
> arrow::ArrayDataVisitor

Here's an example (didn't compile this, but hopefully this gives the idea)

struct BooleanValueVisitor {
  int64_t num_true = 0;
  int64_t num_null = 0;

  Status VisitNull() {
    ++num_null;
    return Status::OK();
  }

  Status VisitValue(bool value) {
    if (value) ++num_true;
    return Status::OK();
  }
};


Status VisitBooleanValues(const Array& arr) {
  BooleanValueVisitor visitor;
  return ArrayDataVisitor<BooleanType>::Visit(*arr.data(), &visitor);
}

If you have a type-parameterized visitor, then you could have

template <typename ArrowType>
Status VisitArrayValues(const Array& arr) {
  MyValueVisitor<ArrowType> visitor;
  return ArrayDataVisitor<ArrowType>::Visit(*arr.data(), &visitor);
}

(FWIW, we developed ArrayDataVisitor primarily for internal library
use and not as a public API)

I would personally try to first use VisitArrayInline if at all
possible since it is simpler

>
> I have a struct:
>
> Struct AnArrayUser
> {
>      template <typename T> arrow::Status Visit(const T &a)
>      {
>            // How to invoke ArrayDataVisitor?
>      }
>
>      void Use(const arrow::Array &a) {arrow::VisitArrayInline(a, this);}
>
>
>      arrow::Status VisitNull() {return arrow::Status::OK();}
>      template <class T> arrow::Status VisitValue(T val) {return arrow::Status::OK();}
> };
>
> Which appears to have it's "Use" method called appropriately. But inside of the Visit
method I have so far been unable to find the incantation to make a call through the ArrayDataVisitor.
I've tried several variations of:
>
> arrow::ArrayDataVisitor<typename T::TypeClass>::Visit(*(array.data()), this);
>
> at the // How to .. line above but can't seem to get it to work. I'm sure I just have
some fundamental misunderstanding of how this is supposed to work. Can someone give me some
guidance?
>
> Matt
>
>
>
> -----Original Message-----
> From: Wes McKinney <wesmckinn@gmail.com>
> Sent: Wednesday, January 22, 2020 12:03 PM
> To: user@arrow.apache.org
> Subject: Re: Converting clickhouse column to arrow array
>
> If you search for "VisitTypeInline" or "VisitArrayInline" in the C++ codebase you can
find numerous examples of where this is used
>
> On Wed, Jan 22, 2020 at 10:58 AM Thomas Buhrmann <thomas.buehrmann@gmail.com> wrote:
> >
> > Hi,
> > I was looking for something similar, but didn't find a good example in the docs
or the source code showing how to use the visitor pattern. It would be great, e.g., to have
an example similar to the "Row to columnar conversion", showing a templated way to read arrow
columns into C++ vectors using the visitor pattern, and without implementing a separate reader
function for each arrow type. Would that be possible?
> >
> > Many thanks,
> > Thomas
> >
> > On Wed, 22 Jan 2020 at 17:13, Wes McKinney <wesmckinn@gmail.com> wrote:
> >>
> >> hi Matt,
> >>
> >> I recommend you use the visitor pattern combined with the
> >> arrow::TypeTraits that we provide
> >>
> >> https://clicktime.symantec.com/38JEFUTGByJzrxbCs1aM2Mn7Vc?u=https%3A%
> >> 2F%2Fgithub.com%2Fapache%2Farrow%2Fblob%2Fmaster%2Fcpp%2Fsrc%2Farrow%
> >> 2Ftype_traits.h
> >>
> >> You'll need to provide a compile-time mapping from Clickhouse types
> >> to Arrow types, but then you can statically access the correct
> >> builder type at compile time
> >>
> >> using ArrowType = typename CHToArrowType<CHType>::ArrowType; using
> >> BuilderType = typename TypeTraits<ArrowType>::BuilderType;
> >>
> >> ...
> >>
> >> or similar. In cases where the exported Clickhouse data does not have
> >> an associated AppendValues method in Arrow you may have to write a
> >> special case (please open JIRA issues if you think there should be
> >> more AppendValues methods)
> >>
> >> Thanks
> >>
> >> On Wed, Jan 22, 2020 at 7:44 AM Calder, Matthew <mcalder@xbktrading.com>
wrote:
> >> >
> >> > Hi,
> >> >
> >> >
> >> >
> >> > I am interfacing arrow to a Clickhouse database using their c++ client.
Both arrow and CH have generic array-like classes with the element data type internalized.
Ideally, I would like to be able to write something like:
> >> >
> >> >
> >> >
> >> > arrow::Array a = SomeConversionInvocation(clickhouse::Column c);
> >> >
> >> >
> >> >
> >> > Where the array and column have the same element type (int, double, string,
…) but the code is generic to the specific type.
> >> >
> >> >
> >> >
> >> > I can do this by explicitly handling specific types through template specialization
but I thought that since arrow already has pretty generic type handling through its templates,
and clickhouse also has similar capability there ought to be a more seamless way to do the
conversion. Zero copy would probably be a lot to ask, but something short of template specializations
for every type is what I am aiming for.
> >> >
> >> >
> >> >
> >> > I currently do explicit type specialization. For example I have functions
like:
> >> >
> >> >
> >> >
> >> > inline std::shared_ptr<arrow::Array> makeArray(const
> >> > std::vector<double> &v)
> >> >
> >> > {
> >> >
> >> >     arrow::DoubleBuilder builder;
> >> >
> >> >     builder.AppendValues(v);
> >> >
> >> >     std::shared_ptr<arrow::Array> array;
> >> >
> >> >     builder.Finish(&array);
> >> >
> >> >     return array;
> >> >
> >> > }
> >> >
> >> >
> >> >
> >> > inline std::shared_ptr<arrow::Array> makeArray(const
> >> > std::vector<int> &v)
> >> >
> >> > {
> >> >
> >> >     arrow::Int32Builder builder;
> >> >
> >> >     builder.AppendValues(v);
> >> >
> >> >     std::shared_ptr<arrow::Array> array;
> >> >
> >> >     builder.Finish(&array);
> >> >
> >> >     return array;
> >> >
> >> > }
> >> >
> >> >
> >> >
> >> > Which I suspect is unnecessarily explicit. Is there a more generic way
of handling the variety of underlying array element data types when constructing arrow::Array
objects? And can someone point me to examples that interface arrow to another similarly generically
typed library (doesn’t have to be clickhouse). Thanks for any guidance.
> >> >
> >> >
> >> >
> >> > Matt
> >> >
> >> >
> >> >
> >> >
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